1. Field of Invention
The present invention relates to a method of patterning a substrate. In particular, the method of the present invention relates to patterning substrates to provide electronic devices such as thin film transistors (TFTs) and/or electro-optic devices thereon.
2. Description of Related Art
Photolithography is presently widely used for the mass production of electronic devices and achieves very high resolution and registration. In photolithography, a spin-coated photoresist layer is provided on a substrate and is exposed by blue or ultraviolet light with an aligner or stepper, which aligns patterns on a master, comprising a mask or reticule, with the substrate. The exposed photoresist is then developed to provide patterns of the photoresist on the substrate. This is generally followed by an etching or deposition process to pattern an objective material. The resolution achieved by photolithography is determined by the wavelength of the exposure light and the optics of the aligner or stepper.
Presently, this photolithographic technique is used not only for small-sized integrated circuits but also for very large active-matrix displays. For example, thin film transistor (TFT) arrays for active-matrix liquid crystal display (LCD) panels require substrates larger than 50 cm2. Particularly high resolution and registration are required for producing arrays of TFTs in LCD panels, since the channel lengths of the TFTs should ideally be lower than 20 μm. However, it has been found that such large substrates tend to exhibit bending, presenting difficulties in providing sufficiently accurate resolution and registration. Furthermore, the photolithographic process must be performed several times to make a complete device and this presents further difficulties in repeating registration with sufficient accuracy. However, manufacturers usually use a single aligner, having sufficiently high resolution and very precise registration mechanism, not only for the formation of channels but also for the other patterning steps. Such an alignment system is expensive. Moreover, processes using such an aligner are also expensive, thereby raising the manufacturing cost of LCD panels.
In order to reduce the cost entailed by the use of photolithography, a variety of non-photolithographic patterning processes have been proposed. For example, micro-contact printing and micro-moulding techniques have been found to be capable of patterning feature sizes down to 1 μm. These techniques use elastic rubber stamps for printing so as to provide good contact between the stamp and a substrate. Due to its elasticity, however, the stamp becomes distorted, which makes it difficult to align the master stamp with the patterns on a substrate. Thus, these techniques have the significant disadvantage of difficulties in accurate registration, especially with large substrates, such as those used for LCD panels.
Inkjet printing technology is now used widely for personal printing. It achieves a very high quality of print, approaching photographic grade. Inkjet printing has also proved to be a promising technique for manufacturing electronic devices such as colour filters for liquid crystal displays and full-colour electroluminescent displays. To achieve such electroluminescent displays, different conjugated polymers are deposited using an inkjet printing technique to provide three colours (blue, green, and red) in the display.
Inkjet techniques had previously been regarded as being comparatively low-resolution patterning techniques and it had therefore previously been thought that inkjet printing was unsuitable for producing TFTs. This is because organic polymer TFTs require a channel length of less than 20 μm to achieve a sufficiently high drain current. To produce such a TFT using an inkjet technique, source, drain and gate electrodes must be printed on a substrate. The source and drain electrodes must have a very small gap between them, since this gap defines the channel in the TFT. Since polymer semiconductors have low carrier mobility, this gap should be less than 20 μm, as noted above, in order to achieve practical characteristics.
However, the resolution presently achieved simply by inkjet printing on a solid substrate is not sufficiently high to pattern the source and drain electrodes with a suitably small gap therebetween (channel length), due to fluctuations in the printing process. In particular, the direction of flight of ink droplets is not always completely perpendicular to the face of the nozzle plate of the inkjet print head from which they are ejected, resulting in patterning errors. Furthermore, an ejected droplet spreads on the surface of the substrate onto which it is ejected. The amount the droplet spreads is a function of the surface energies and the interfacial energy of the solid substrate and of the liquid droplet respectively. However, there are fluctuations in the surface energy and the interfacial energy of the solid surface. This results in variations in size of respective droplets deposited on the substrate. Accordingly, the width of the gap between two deposited droplets and hence the channel length of a printed TFT is variable and, in the worst case, short circuits are formed between the source and drain electrodes.
Nonetheless, all-polymer TFTs have previously been fabricated by inkjet deposition. In such fabrication, the source, drain and gate electrodes are formed of a conducting polymer, PEDOT (poly-ethylenedioxythiophene, Baytron P from Bayer AG), and deposited using an inkjet technique. In order to obtain a satisfactory channel width, inkjet printing can be combined with the pre-patterning of wetting properties. This allows control of the flow of ink on the substrate by using a pattern of hydrophilic and hydrophobic substrate regions. As shown in FIG. 8(b), a non-wetting or hydrophobic repelling strip 102 of polyimide (PI) can first be formed on a glass substrate 100 by photolithography, micro-contact printing, micro-moulding printing or photo-induced wettability patterning. This repelling strip 102 defines the channel 106 of the TFT, the width of the strip 102 being the length L of the channel 106, as shown in FIG. 8(a). The remaining area of the substrate 100 is hydrophilic or wetting with respect to a solution of PEDOT. Source and drain electrodes can then be formed by depositing a water-based solution of PEDOT onto the glass substrate using an inkjet print head. The PEDOT solution exhibits relatively high contact angles of around 70° on the PI strip, and small contact angles of less than 20° on the glass region. Thus, when droplets 104 of PEDOT solution are deposited along the strip 102, the droplets 104 spread over the substrate 100 but are repelled by the strip 102. The solution 104 on the substrate 100 is therefore confined from spreading over the repelling strip 102, but instead aligns along the side of the strip 102. Using this self-aligning mechanism, source 108 and drain 110 electrodes with a channel length L shorter than 20 μm and as low as 5 μm can be achieved.